A method and system for in-situ detection of the thickness of an aircraft stealth coating under unknown base material conditions

Abstract

The application discloses a kind of unknown parent material conditions under aircraft stealth coating thickness in-situ detection method and system, first for aircraft limited kind known parent material and standard stealth coating, establish reference database of thickness-eddy current response calibration curve containing parent material benchmark electromagnetic response spectrum, stripping coating intrinsic electromagnetic characteristic influence;Again, the multi-frequency eddy current probe is obtained to the measured site multi-frequency complex impedance response signal, it is matched with the parent material benchmark spectrum in database to identify parent material type;Finally, the thickness calibration curve of corresponding parent material-coating combination is called, and the actual thickness of coating is obtained by substituting response signal solution.The technical scheme of the application only needs a whole eddy current detection probe on hardware, and simultaneously realizes the parent material matching classification identification and stealth coating thickness detection of stealth aircraft, and is portable, intelligent, convenient and practical.

Description

Technical Field

[0001] This invention relates to the field of nondestructive testing technology, specifically to a coating thickness measurement method and system based on multi-frequency eddy current technology, which is particularly suitable for in-situ, rapid, and accurate detection of aircraft stealth coating thickness in the case of unknown substrate material. Background Technology

[0002] Stealth coating is a core technology for achieving radar stealth in modern military aircraft, and its thickness directly determines the aircraft's stealth performance. During long-term service, the stealth coating is prone to wear and tear due to airflow erosion, temperature changes, and maintenance operations, resulting in thickness reduction. Therefore, condition-based inspection of the stealth coating thickness of in-service aircraft is an important part of aircraft maintenance.

[0003] Currently, the eddy current method has become a commonly used method for coating thickness measurement due to its advantages such as non-contact operation, fast detection speed, and sensitivity to coatings. However, the traditional single-frequency eddy current thickness measurement technology has a fundamental limitation: the measurement accuracy is highly dependent on the accurate knowledge of the electromagnetic properties (conductivity, permeability, etc.) of the substrate material, and the eddy current signal is affected by both the coating thickness and the properties of the substrate material.

[0004] At aircraft maintenance sites, inspectors often struggle to quickly and accurately determine the specific type of base material for the area being inspected. Furthermore, stealth coatings frequently contain metallic particles or magnetic materials, whose inherent electromagnetic properties can cause additional interference to the eddy current field. The combination of these two factors—"unknown base material" and "electromagnetic interference from the coating"—makes it impossible for traditional eddy current methods to achieve accurate thickness measurement in this scenario.

[0005] In the existing technology, although multi-frequency eddy current technology has been applied to suppress the lift-off effect or material sorting, there is no mature technical solution that systematically applies it to solve the practical industrial problem of "rapidly and accurately measuring the thickness of aircraft stealth coating under the condition of unknown base material and electromagnetic interference of coating". The pain point of stealth coating thickness measurement at aircraft maintenance site has not been effectively solved.

[0006] To address the above-mentioned shortcomings, the present invention adopts the following technical solution. Summary of the Invention

[0007] The purpose of this invention is to provide a method and system for in-situ detection of aircraft stealth coating thickness under unknown base material conditions. This method enables rapid and accurate in-situ measurement of aircraft stealth coating thickness without prior knowledge of the substrate material type and effectively eliminates electromagnetic interference from the stealth coating itself, thus meeting the in-service testing requirements of aircraft hangars and field applications.

[0008] The core technical approach of this invention is "sorting first, then measuring thickness." It requires only an eddy current testing probe. First, multi-frequency eddy current technology is used to intelligently identify the type of base material. Then, a matching calibration curve is retrieved from a pre-set reference database to calculate the coating thickness. The specific technical solution is as follows: A method for in-situ detection of aircraft stealth coating thickness under unknown base material conditions includes the following steps: S1. Establish a reference database: For a limited number of known aircraft substrate materials and standard stealth coatings, obtain the reference electromagnetic response spectrum of each substrate material at multiple frequencies through experimental measurement or theoretical modeling, and establish a thickness-eddy current response calibration curve for each type of "substrate material-coating" combination after removing the influence of the intrinsic electromagnetic properties of the coating, and store it in the database. S2. On-site identification and measurement: Use a multi-frequency eddy current probe to detect the stealth coating of the aircraft under test and obtain its complex impedance response signal at multiple set frequencies. S3. Intelligent matching of base material type: The multi-frequency response signal obtained in step S2 is matched and analyzed with the reference electromagnetic response spectra of various known base materials stored in the database in step S1 to identify and determine the base material type of the current test part. S4. Coating thickness calculation: Based on the base material type identified in step S3, the corresponding thickness-calibration curve is retrieved from the database, and the response signal obtained in step S2 is substituted to calculate the actual thickness value of the current stealth coating.

[0009] In step S1, the known limited types of aircraft matrix materials are a limited set of options determined based on aircraft manufacturing information, including but not limited to one or more of aluminum alloys, titanium alloys, and high-strength steels.

[0010] Furthermore, in step S1, the method for establishing the "thickness-calibration curve after stripping the influence of the intrinsic electromagnetic properties of the coating" is as follows: on a standard test block, the stealth coating with a known thickness gradient is prepared, and its multi-frequency eddy current response is measured; by inverting the electromagnetic field model or comparing it with the response of the uncoated base material, the inherent influence of the coating material itself on the eddy current field is separated and subtracted, thereby establishing a pure calibration curve that only reflects the thickness change.

[0011] Furthermore, the matching analysis in step S3 employs at least one of the following algorithms: least squares method, correlation coefficient method, or machine learning-based pattern classification algorithm. This determines the type of base material by comparing the similarity between the trajectory or feature vector of the multi-frequency response signal on the impedance plane and the reference spectrum in the database. Additionally, the matching of base material types is performed based on one of the following electromagnetic characteristics: conductivity differences: the impedance point of high-conductivity alloys (such as copper) is located on the right side of the semicircle, while that of low-conductivity alloys (such as titanium alloys) is to the left; magnetic permeability influence: ferromagnetic materials (such as carbon steel) have complex impedance trajectories due to conductivity variations, requiring analysis in conjunction with characteristic frequencies.

[0012] Furthermore, step S4, calculating the actual thickness of the stealth coating, includes calculating the impedance response to thickness changes through matching analysis, and comparing matching analysis charts for non-ferromagnetic and ferromagnetic materials to calculate the coating thickness for different materials. For non-ferromagnetic materials, chart characteristics show that as the thickness decreases, the impedance point moves along a semi-circular trajectory towards decreasing resistance, and the normalized resistance of key parameters shows a linear relationship with thickness; high frequencies are more sensitive to thinner layers. For ferromagnetic materials, after magnetic saturation treatment to stabilize the permeability, thickness changes are manifested as the impedance point shifting to the upper right.

[0013] Furthermore, the matching analysis in step S3 also includes the identification and differentiation of defects in the base material, including the base material matching analysis after the calculation and identification analysis of surface defects and internal defects. For example, surface defects (such as cracks, scratches, etc.) move from the stable position when there are no defects to the direction of increasing reactance through the impedance point of the graph features, forming a significant offset trajectory, and the change in reactance is significantly greater than the change in resistance in the key parameters. The deeper the defect, the greater the offset amplitude. In addition, due to the skin effect, the signal offset of internal defects is weaker and the trajectory is close to the origin, requiring low-frequency detection to enhance the penetration depth.

[0014] Furthermore, the multi-frequency eddy current probe operates in the frequency range of 100 Hz to 10 MHz, and includes at least one low frequency sensitive to the properties of the base material and one high frequency sensitive to the coating thickness.

[0015] This invention applies eddy current detection technology to the analysis of alloy and coating combinations in stealth aircraft. Frequency selection and sensitivity design must consider both material properties and deep detection. The core method involves selecting a high-frequency range, applicable to surface coating thickness and scenarios involving defect detection (such as cracks and spalling), utilizing the skin effect to focus on the surface layer; and in stealth coating adaptation for conductive coatings (such as metal nanoparticle doping), suppressing high-frequency signal attenuation. The low-frequency range of eddy current signals is suitable for matching detection of the fuselage alloy substrate (such as titanium alloy), enhancing penetration depth. Optimization of the fuselage and coating composite structure involves separating coating and substrate signals through multi-frequency scanning. Sensitivity design methods and probe optimization can utilize differential probes to eliminate background noise and improve the signal-to-noise ratio of small defects; or array probes to cover complex surfaces and achieve full-field imaging. Signal processing uses phase analysis to distinguish between changes in coating conductivity and alloy material matching or defects. In addition, special considerations for stealth materials include coating conductivity adaptation to non-conductive coatings: low-frequency eddy current + ultrasonic combined detection is adopted; for conductive coatings (such as ferrite): the frequency is adjusted to avoid the magnetic loss frequency band.

[0016] This invention also discloses an in-situ detection system for the thickness of aircraft stealth coatings under unknown base material conditions, comprising: 1. Multi-frequency eddy current detector (1): It has a built-in multi-frequency excitation source (11) and signal acquisition and processing module (12); II. Probe (2): Connected to the detector, used to emit multi-frequency magnetic fields and receive induced signals; 3. Intelligent processing unit (3): Located in the detector or in the intelligent terminal (5) connected to it, the intelligent processing unit (3) stores a reference database (31) for data information matching and integrates a base material matching module (32) and a thickness calculation module (33); IV. Human-computer interaction interface (4): used to display the identified base material type, the calculated coating thickness value and the corresponding confidence level.

[0017] The above system modules are combined into a portable device, which uses an integrated eddy current detection probe to simultaneously achieve the matching, classification and identification of base materials for stealth aircraft and the detection and monitoring of stealth coating thickness. It is suitable for portable and rapid use in aircraft hangars or field in-service inspection environments.

[0018] Furthermore, the base material matching module (32) includes an impedance plane image analysis module (321) and a multi-frequency response signal comparison module (322). The base material alloy type is determined by comparing the similarity between the trajectory or feature vector of the multi-frequency response signal on the impedance plane and the reference spectrum in the database. The base material alloy type includes, but is not limited to, one or more of aluminum alloys, titanium alloys, and high-strength steel.

[0019] Furthermore, the thickness calculation module (33) includes an impedance response matching calculation module (331) and a magnetic material comparison analysis calculation module, which are used to calculate the actual thickness value of the stealth coating, including the impedance response of thickness change by matching calculation, and the comparison analysis of matching analysis diagrams for non-ferromagnetic materials and ferromagnetic materials to calculate the coating thickness of different materials.

[0020] Furthermore, the intelligent processing unit (3) also includes a base material defect analysis module (34) for distinguishing and identifying defects during the base material matching analysis process. The base material defect analysis module (34) also includes an impedance defect analysis module (341) and an internal and external defect analysis module (342) for identifying and distinguishing base material defects and calculating and identifying surface and internal defects during the base material matching analysis.

[0021] Based on the above technical solution, this invention has the following beneficial effects: 1. It breaks through the technical limitation of traditional eddy current thickness measurement requiring "the base material must be known," and innovatively integrates multi-frequency eddy current material sorting technology into the thickness measurement process, eliminating the need for inspection personnel to pre-input base material information, greatly improving the adaptability and ease of operation of on-site inspection; 2. By establishing a "pure" thickness calibration curve that removes the influence of the intrinsic electromagnetic properties of the coating through pre-calibration, it effectively eliminates the interference of functional fillers in stealth coatings on the eddy current field, significantly improving the absolute accuracy of coating thickness measurement; 3. It achieves in-situ rapid and accurate measurement. The entire process of base material identification and thickness calculation can be automatically completed within seconds. The operation is simple, the results are intuitive, and it is suitable for the rapid inspection needs of aircraft field or hangar, providing efficient data support for maintenance decisions; 4. It is specifically optimized for the engineering scenario of "limited known base materials + specific stealth coatings" in aircraft manufacturing. The technical solution of this invention only requires one integrated eddy current detection probe in terms of hardware, simultaneously realizing the base material matching and classification identification of stealth aircraft and the detection of stealth coating thickness. It is portable, intelligent, convenient, and has high practical value. Attached Figure Description

[0022] Figure 1 This is a schematic diagram of a method flow according to one embodiment of the present invention; Figure 2 This is a schematic diagram illustrating the detection method and analysis calculation of one embodiment of the present invention; Figure 3 This is a schematic diagram of a multi-frequency eddy current impedance plane according to one embodiment of the present invention; Figure 4 This is a schematic diagram of base material matching on a multi-frequency eddy current impedance plane according to one embodiment of the present invention. Figure 5 This is a schematic diagram of the system structure according to one embodiment of the present invention. Detailed Implementation

[0023] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. The following examples are for illustrative purposes only and do not constitute a limitation on the scope of protection of the present invention.

[0024] Reference Figure 1-5 This invention discloses a method and system for in-situ detection of aircraft stealth coating thickness under unknown base material conditions. Through a constructed matching reference database, only an integrated eddy current detection device is needed for data detection. After intelligently analyzing multi-frequency eddy current detection data to identify the base material type, the system retrieves the matching calibration curve from the preset matching reference database to calculate the coating thickness. This achieves portable, intelligent, and rapid detection and analysis even with unknown base material and coating of stealth aircraft. The core technology involves selecting the optimal frequency for coating thickness detection after identifying the unknown base material through multi-frequency eddy current matching, enabling simple and intelligent detection. A preferred implementation is as follows: like Figure 1 As shown in the flowchart of the Chinese method, the in-situ detection method for aircraft stealth coating thickness under unknown base material conditions has the following four steps: S1 Establishing a reference database, S2 On-site identification and measurement, S3 Intelligent matching of base material type, and S4 Coating thickness calculation.

[0025] like Figure 2 and 3 As shown, the first step, S1, establishes a reference database: For a limited number of known aircraft substrate materials and standard stealth coatings, the reference electromagnetic response spectrum of each substrate material at multiple frequencies is obtained through experimental measurement or theoretical modeling. A thickness-eddy current response calibration curve, stripped of the intrinsic electromagnetic properties of the coating, is established for each type of "substrate-coating" combination and stored in the database. There are three main types of stealth aircraft substrate materials: aluminum alloy, titanium alloy, and high-strength steel. A reference database for matching is established, including data information on these three main substrate materials and manufacturing information data for stealth coatings designed for these three substrate materials (this is just one example in one implementation; other substrate materials could include carbon fiber composites, etc.). A multi-frequency eddy current meter is used to measure the complex impedance at different high and low frequencies. In a preferred implementation, the frequencies are 1kHz, 10kHz, 100kHz, and 1MHz, forming eigenvector diagrams Z_A0, Z_B0, and Z_C0, which are then stored in the database.

[0026] like Figure 2As shown, in establishing a reference database, the thickness-calibration curves, which isolate the influence of the intrinsic electromagnetic properties of the coating, are established. On standard test blocks of the same stealth coating model, with thicknesses ranging from 0.1 mm to 2.0 mm (gradient of 0.1 mm), prepared using a spraying process, the impedance characteristics of the eddy current response of all samples at the same four frequency points are measured. Using an analytical electromagnetic field model, the contribution of the coating's intrinsic electromagnetic parameters is subtracted from the measurement signals of the coated samples, resulting in a set of "effective responses" Z_eff(h) that are only related to the physical thickness. For each type of "base material-coating" combination, calibration curves f_A(h), f_B(h), and f_C(h) of Z_eff versus h are fitted and stored in the database.

[0027] like Figure 2 As shown, the second step, S2 on-site identification and measurement, involves using a multi-frequency eddy current probe to detect the stealth coating area of ​​the aircraft under test. The probe is placed at the test point, and the instrument automatically excites and acquires the response signal Z_meas at 1kHz, 10kHz, 100kHz, and 1MHz.

[0028] like Figure 2 and Figure 4 As shown in the figure, the third step, S3, intelligent matching of base material type: calculate the normalized correlation coefficient between Z_meas and Z_A0, Z_B0, and Z_C0 in the database. If the correlation coefficient of Z_A0 is the highest (>0.99), the system determines that the current base material is aluminum alloy.

[0029] like Figure 2 As shown, the fourth step, S4, involves calculating the coating thickness: Based on the identified base material type, the f_A(h) calibration curve is retrieved from the database. The accuracy of f_A(h) directly determines the measurement accuracy of the entire system. Since the optimal frequency is related to the coating thickness and electromagnetic properties, after matching the base material type to aluminum alloy, 100kHz is determined to be the optimal frequency. The component in Z_meas that is most sensitive to thickness at 100kHz is substituted into f_A(h), and through automatic table lookup and interpolation calculation, the current coating thickness is calculated to be 0.75mm.

[0030] like Figure 4As shown, in the preferred embodiment of the present invention, the eddy current detection information calculation and analysis, in step S3, the parent material classification and matching analysis employs at least one of the following algorithms: least squares method, correlation coefficient method, or a machine learning-based pattern intelligent recognition classification algorithm. This determines the parent material type by comparing the similarity between the trajectory or feature vector of the multi-frequency response signal on the impedance plane and the reference spectrum in the database. In step S4, coating thickness detection, calculating the actual thickness of the stealth coating includes calculating the impedance response to thickness changes through matching, and comparing and analyzing the matching analysis diagrams of non-ferromagnetic and ferromagnetic materials to calculate the coating thickness of different materials. When the thickness of the non-ferromagnetic stealth coating decreases, its response change on the eddy current detection impedance plane diagram is essentially a direct manifestation of the change in the interaction characteristics between the eddy current field and the material. For example, when the thickness decreases, the impedance point moves along a semi-circular trajectory in the direction of decreasing resistance, and the normalized resistance (R / R0) of the key parameter has a linear relationship with the thickness. For ferromagnetic materials, after magnetic saturation treatment to stabilize the permeability, the thickness change manifests as the impedance point moving to the upper right.

[0031] The matching analysis in step S3 also includes the identification and differentiation of defects in the base material, including the base material matching analysis after the calculation and identification analysis of surface defects and internal defects. For example, surface defects (such as cracks, scratches, etc.) move from the stable position when there are no defects to the direction of increasing reactance through the impedance point of the graph features, forming a clear offset trajectory. In the key parameters, the change in reactance is significantly greater than the change in resistance. The deeper the defect, the greater the offset amplitude. In the graph features of internal defects, due to the skin effect, the signal offset is weaker and the trajectory is close to the origin. Low-frequency detection is required to enhance the penetration depth.

[0032] In a preferred embodiment, this invention applies multi-frequency eddy current detection technology to the analysis of alloy and coating combinations in stealth aircraft. Frequency selection and sensitivity design must balance material properties and deep penetration. The core method involves selecting a frequency range between 100kHz and 10MHz, suitable for surface coating thickness and defect detection (such as cracks and spalling), utilizing the skin effect (δ∝1 / √f) to focus on the surface layer. It also addresses the need to suppress high-frequency signal attenuation in stealth coating adaptation for conductive coatings (such as metal nanoparticle doping). Low-frequency eddy current signals (1kHz-100kHz) are suitable for matching detection of fuselage alloy substrates (such as titanium alloys), enhancing penetration depth (δ up to millimeters). Optimization of the fuselage and coating composite structure involves separating coating and substrate signals through multi-frequency scanning. Sensitivity design methods and probe optimization can utilize differential probes to eliminate background noise and improve the signal-to-noise ratio of minute defects (such as coating thickness variations of ±5μm); or array probes to cover complex curved surfaces and achieve full-field imaging. Signal processing uses phase analysis to distinguish between changes in coating conductivity and alloy material matching or defects. Special considerations for stealth materials include coating conductivity adaptation for non-conductive coatings: low-frequency eddy current + ultrasonic combined detection; for conductive coatings (such as ferrites): adjusting the frequency to avoid magnetic loss bands.

[0033] like Figure 5 As shown in the figure, an in-situ detection system for aircraft stealth coating thickness under unknown base material conditions is also disclosed in this embodiment of the invention, comprising: Multi-frequency eddy current detector 1: It has a built-in multi-frequency excitation source 11 and a signal acquisition and processing module 12; Probe 2: Connected to the detector, used to emit multi-frequency magnetic fields and receive induced signals; Intelligent processing unit 3: Located in the detector or in the intelligent terminal (5) connected to it, the intelligent processing unit 3 stores a reference database 31 for data information matching, and integrates a base material matching module 32 and a thickness calculation module 33; In a preferred embodiment, the reference database 31 is composed of the detection data of the multi-frequency eddy current detection standard test block in step S1 of the above method of the present invention (multi-frequency eddy current detector 1 and probe 2), which are stored in the base material matching information database 311 and the thickness calibration database 312 respectively. In a preferred embodiment, the base material matching information database 311 collects and stores the base material characteristic texture data, and the thickness calibration database 312 collects and stores the coating thickness standard curve data.

[0034] Human-machine interface 4: used to display the identified base material type, the calculated coating thickness value and the corresponding confidence level. In a specific embodiment, the human-machine interface (4) can be the display screen of the eddy current detector or any smart terminal such as a mobile phone, PC, tablet, industrial computer, etc., to realize a lightweight host and high-performance computing combination.

[0035] The above system modules are combined into a portable device, which uses an integrated eddy current detection probe to simultaneously achieve the matching, classification and identification of base materials for stealth aircraft and the detection and monitoring of stealth coating thickness. It is suitable for portable and rapid use in aircraft hangars or field in-service inspection environments.

[0036] The base material matching module 32 includes an impedance plane image analysis module 321 and a multi-frequency response signal comparison module 322. It determines the base material alloy type by comparing the similarity between the trajectory or eigenvector of the multi-frequency response signal on the impedance plane and a reference spectrum in the database. The base material alloy type includes, but is not limited to, one or more of aluminum alloys, titanium alloys, and high-strength steel.

[0037] The thickness calculation module 33 includes an impedance response matching calculation module 331 and a magnetic material comparison analysis calculation module 332, which are used to calculate the actual thickness value of the stealth coating, including the impedance response of thickness change through matching calculation, and the comparison analysis of matching analysis diagrams for non-ferromagnetic and ferromagnetic materials to calculate the coating thickness of different materials.

[0038] The intelligent processing unit 3 also includes a base material defect correction and analysis module 34 for distinguishing and identifying defects during the base material matching analysis process. The base material defect analysis module (34) also includes an impedance defect analysis module 341 and an internal and external defect analysis module 342, which are used for identifying and distinguishing base material defects and calculating and identifying surface and internal defects during the base material matching analysis.

[0039] The above is one embodiment of the present invention. Furthermore, it should be noted that any equivalent or simple variations made to the structure, features, and principles described in this patent concept are included within the scope of protection of this patent.

Claims

1. A method for in-situ detection of aircraft stealth coating thickness under unknown base material conditions, characterized in that... Includes the following steps: S1. Establish a reference database: For a limited number of known aircraft substrate materials and standard stealth coatings, obtain the reference electromagnetic response spectrum of each substrate material at multiple frequencies through experimental measurement or theoretical modeling, and establish a thickness-eddy current response calibration curve for each type of "substrate material-coating" combination after removing the influence of the intrinsic electromagnetic properties of the coating, and store it in the database. S2. On-site identification and measurement: Use a multi-frequency eddy current probe to detect the stealth coating of the aircraft under test and obtain its complex impedance response signal at multiple set frequencies. S3. Intelligent matching of base material type: The multi-frequency response signal obtained in step S2 is matched and analyzed with the reference electromagnetic response spectra of various known base materials stored in the database in step S1 to identify and determine the base material type of the current test part. S4. Coating thickness calculation: Based on the base material type identified in step S3, the corresponding thickness-calibration curve is retrieved from the database, and the response signal obtained in step S2 is substituted to calculate the actual thickness value of the current stealth coating.

2. The method for in-situ detection of aircraft stealth coating thickness under unknown base material conditions according to claim 1, characterized in that, The method for establishing the "thickness-calibration curve with the influence of intrinsic electromagnetic properties of the coating stripped" in step S1 is as follows: On a standard test block, the stealth coating with a known thickness gradient is prepared, and its multi-frequency eddy current response is measured; by inverting the electromagnetic field model or comparing it with the response of the uncoated base material, the inherent influence of the coating material itself on the eddy current field is separated and subtracted, thereby establishing a pure calibration curve that only reflects the thickness change.

3. The method for in-situ detection of aircraft stealth coating thickness under unknown base material conditions as described in claim 1, characterized in that... The matching analysis in step S3 employs at least one of the following algorithms: least squares method, correlation coefficient method, or machine learning-based pattern classification algorithm. By comparing the similarity between the trajectory or feature vector of the multi-frequency response signal on the impedance plane and the reference spectrum in the database, the type of parent material is determined.

4. The method for in-situ detection of aircraft stealth coating thickness under unknown base material conditions as described in claim 1, characterized in that... The step S4, which calculates the actual thickness of the stealth coating, includes calculating the impedance response of thickness changes by matching, and comparing and analyzing the matching analysis diagrams of non-ferromagnetic and ferromagnetic materials to calculate the coating thickness of different materials.

5. The method for in-situ detection of aircraft stealth coating thickness under unknown base material conditions according to claim 1, characterized in that... The matching analysis in step S3 also includes the identification and differentiation of defects in the base material, including the base material matching analysis after the calculation, identification and analysis of surface defects and internal defects.

6. A method for in-situ detection of aircraft stealth coating thickness under unknown base material conditions according to any one of claims 1 to 5, characterized in that... The multi-frequency eddy current probe operates in the frequency range of 100 Hz to 10 MHz, and includes at least one low frequency that is sensitive to the properties of the base material and one high frequency that is sensitive to the coating thickness.

7. An in-situ detection system for the thickness of aircraft stealth coating under unknown base material conditions, characterized in that... include: Multi-frequency eddy current detector (1): It has a built-in multi-frequency excitation source (11) and signal acquisition and processing module (12); Probe (2): Connected to the detector, used to emit multi-frequency magnetic fields and receive induced signals; Intelligent processing unit (3): Located in the detector or in the intelligent terminal (5) connected to it, the intelligent processing unit (3) stores a reference database (31) for data information matching and integrates a base material matching module (32) and a thickness calculation module (33); Human-computer interaction interface (4): used to display the identified base material type, the calculated coating thickness value and the corresponding confidence level.

8. The in-situ detection system for aircraft stealth coating thickness under unknown base material conditions according to claim 7, characterized in that... The base material matching module (32) includes an impedance plane image analysis module (321) and a multi-frequency response signal comparison module (322). The base material alloy type is determined by comparing the similarity between the trajectory or feature vector of the multi-frequency response signal on the impedance plane and the reference spectrum in the database.

9. The in-situ detection system for aircraft stealth coating thickness under unknown base material conditions according to claim 7, characterized in that... The thickness calculation module (33) includes an impedance response matching calculation module (331) and a magnetic material comparison analysis calculation module, which are used to calculate the actual thickness value of the stealth coating, including the impedance response of thickness change by matching calculation, and the thickness of coatings of different materials by comparing the matching analysis diagrams of non-ferromagnetic materials and ferromagnetic materials.

10. The in-situ detection system for aircraft stealth coating thickness under unknown base material conditions according to claim 9, characterized in that... The intelligent processing unit (3) also includes a base material defect analysis module (34) for distinguishing and identifying defects during the base material matching analysis process.

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